1. Field of the Invention
This invention relates to a method and apparatus for recording and an apparatus for reproducing digital video signals. More particularly, it relates to a method for recording digital video signals whereby data produced by encoding video signals by e.g., the so-called MPEG system can be directly recorded and reproduced pictures having a superior quality may be obtained due to variable-speed reproduction.
2. Description of the Related Art
Developments of a digital video tape recorder (digital VTR), in which video signals are converted into digital signals, discrete cosine transformed and variable length encoded by e.g., Huffman encoding by way of data compression, and in which the resulting digital video signals are recorded on a magnetic tape by a rotary head in accordance with an azimuth recording system, are currently proceeding. In such digital VTR, the mode of recording video signals of the current television system, such as the NTSC system, referred to herein as SD mode, or the mode of recording of the high-definition television signals (HDTV signals), referred to herein as the HD mode, may be set.
In the SD mode, video signals are recorded after compression to digital video signals of approximately 25 Mbps, whereas, in the HD mode, HDTV signals are recorded after compression to digital video signals of approximately 50 Mbps.
With the conventional digital VTR, it has been envisaged to record input digital video signals (input data) directly on a magnetic tape and to reproduce and directly output data recorded on the magnetic tape. This has an advantage in that, by adding the function of directly recording/reproducing digital video signals (data) to the conventional digital VTR, the necessity of temporarily decoding input digital video signals for reproducing e.g., HDTV signals and re-encoding the HDTV signals in accordance with a pre-set encoding system for recording on the magnetic tape may be eliminated, thus obviating additional hardware.
Specifically, if the digital VTR is fed with digital video signals obtained on encoding video signals in accordance with the MPEG system, that is a moving picture encoding system standardized in the Work Group (WG) 11 of the Sub-Committee of Joint Technical Committee (JTC) of the International Standardization Organization (ISO) and International Electric Committee (IEC), or with digital video signals reproduced from an optical disc, it is very convenient if the digital VTR has the function of directly recording/reproducing these digital video signals.
The Advanced Television system (ATV system), which is a digital broadcasting employing the above MPEG system as the encoding system, is now explained. FIG. 12 shows, in a block diagram, the construction of the transmission system of the ATV system. In FIG. 12, the numerals 101 and 102 denote a video compression encoder and an audio encoder, respectively. Video signals of the HDTV system are fed via an input terminal 103 to the video compression encoder 101, while audio signals are fed from an input terminal 104.
The video compression encoder 101 encodes the input HDTV signals in accordance with the MPEG system for data compression. Thus the video compression encoder 101 encodes the input HDTV signals by a high efficiency encoding system combined from DCT and motion compensation predictive coding for data compression.
The video compression encoder 101 outputs data of an intra-field or intra-frame coded picture, that is I-picture, data of a forward prediction coded picture or P-picture and data of a bidirectionally prediction coded picture or B-picture, in a pre-set sequence, as shown in FIG. 13. In the I-picture, DCT is applied independently without employing correlation with other pictures. In the P-picture, motion compensated prediction coding is done from previous I-picture or P-picture and the difference signal, or so-called prediction error, is discrete cosine transformed. In the B-picture, motion compensated predictive coding is done from the forward and backward I-picture or P-picture and the difference signals is similarly discrete cosine transformed. The period of appearance of the I-picture is termed a group of pictures (GOP). In the present case, M and N are set so that M=3 and N=9.
A transport encoder 106 generates packets from video data encoded by the video compression encoder 101, audio data encoded by the audio encoder 104 and the ancillary information supplied to an input terminal 107.
FIG. 14 shows packet configuration. As shown therein, a transmitted packet has a packet length of 188 bytes. At the leading end of the packet is provided a linking header having a fixed 4-byte length and an adaptation header of a variable length, followed by transmission data consisting of video or audio data.
In FIG. 12, the packet generated by the transport encoder 106 is supplied to a channel modulator 108 which modulates the packet using a carrier of a pre-set frequency. An output of the channel modulator 108 is issued at an output terminal 109.
It is possible with the ATV system to transmit HDTV signals at a rate of e.g., 19 Mbps by the above-described picture compression. This is lower than the recording rate in the SD mode of the digital VTR (about 25 Mbps). Thus the signals (data) transmitted by the ATV system can be directly recorded with the SD mode of the digital VTR. If the transmitted signals are directly recorded by the digital VTR, there is no necessity of decoding HDTV signals from the transmitted signals and entering the decoded signals into the digital VTR thus obviating additional hardware. In addition, since recording may be done with the SD mode, the recording time may be prolonged.
However, if the ATV signals are directly recorded with the SD mode on the digital VTR, variable-length reproduction cannot be achieved for the following reason.
That is, compression (encoding) pursuant to the MPEG system is done with the ATV system, as explained above. With this system, data of an intra-field or intra-frame coded picture, that is I-picture, data of a forward prediction coded picture or P-picture and data of a bidirectionally prediction coded picture or B-picture are transmitted, as also explained above. During variable-speed reproduction, data of continuous pictures cannot be produced, because the head traverses the track on the magnetic tape. If the data of the continuous pictures is not produced, data of the P-picture and the B-picture cannot be decoded. It is only the intra-picture encoded data, that is I-picture data, that can be decoded. Thus, during variable-speed reproduction, the variable-speed reproduction is enabled by employing solely the data of the I-picture among the reproduced data.
However, if the signals transmitted in accordance with the ATV system is directly recorded on the digital VTR, the packet carrying the I-picture cannot be sufficiently picked up during the variable-length reproduction. On the other hand, it is indefinite in which position the I-picture data is recorded. Thus it occurs frequently that data of the I-picture corresponding to a pre-set portion of the frame during variable-length reproduction is missing and the frame of such portion cannot be updated for a certain time thus deteriorating the picture quality.
The present Assignee previously proposed an arrangement in which data of an I-picture is extracted from an input signal bitstream of the ATV system and recorded in an area for variable speed reproduction, while the signals of the ATV system are directly recorded in other areas, such as a video sector. The area for variable speed reproduction is an area from which reproduction may be done during variable speed reproduction. In this case, data from the area for variable speed reproduction is reproduced during variable speed reproduction and a frame is formed from the data of the I-picture reproduced from this area.
The present Assignee also proposed the relevant technology in the following two Applications:
i) European Patent Publication No. 0627855 (published data, 1994.12.07) PA1 ii) European Patent Publication No. 0650296 (published data, 1995.04.26.
The U.S. application Ser. Nos. 08/327,370 (U.S. Pat. No. 5,684,917) and 08/868,370, respectively, corresponding to these European Patent Publications are now pending at the US Patent Office. The above-mentioned applications, owned by the present Assignee, are hereby incorporated by reference.
However, the reproducible area at the time of variable-speed reproduction is changed with the speed of the variable-speed reproduction. Thus it is difficult to set multiple variable playback speeds. For example, if data for variable-speed reproduction is recorded in a common area reproducible at 17-tuple, 9-tuple or quadruple speed, variable-speed reproduction at the quadruple speed, 9-tuple speed and 17-tuple speed becomes possible, while it is difficult to achieve variable-speed reproduction at any other speed.